A distributed power supply such as a solar cell or a fuel cell includes a power conditioner configured to convert power in order to adapt a frequency and a voltage to a commercial system power supply for use in interconnection with the commercial system power supply. A hybrid distributed power supply obtained by including a storage battery to such a distributed power supply has also been attracting attention in recent years.
A power conditioner applicable to a hybrid distributed power supply includes a DC/DC converter configured to adjust a DC voltage of power generated by the distributed power supply to a predetermined DC voltage value, a bidirectional DC/DC converter (hereinafter, simply referred to as a “DC converter”) configured to charge or discharge a storage battery, an inverter configured to convert a DC bus voltage connected with the bidirectional DC/DC converter to an AC voltage, an LC filter configured to remove a high frequency component from output of the inverter, and the like.
In a case where a shunt fault occurs on a distribution line for a distributed power supply in grid connected operation, or power transmission from a substation to the distribution line stops due to planned power outage or the like, that is, an isolated operation state is established, in order to prevent influence on operation of a sectionalizing switch and secure safety during handling of the distribution line, the power conditioner separates the distributed power supply from the distribution line.
The power conditioner converts DC power of the distributed power supply to AC power and controls to supply the power to an independent power system with no interconnection with a commercial power system.
The power conditioner includes a control unit configured to control a distributed power supply. The control unit includes a current control block configured to control an inverter to output an AC current in synchronization with a phase of a commercial power system voltage at commercial grid connected operation, and a voltage control block configured to control the inverter for output of a voltage at a predetermined level provided to a stand-alone power system upon power system separation.
The provided voltage at the predetermined level corresponds to a voltage for low-voltage customers prescribed in Electricity Business Act, Article 26 and the Ordinance for Enforcement of the Act, Article 44 in Japan, and falls within 101±6 V with respect to a standard voltage 100 V and within 202±20 V with respect to a standard voltage 200 V.
Patent Document 1 discloses an inverter device for a distributed power supply, configured to suppress, within a predetermined allowable range, variation in output voltage of the inverter device due to load variation at grid independent operation.
The inverter device includes a voltage control circuit provided with a first determination circuit configured to determine an output voltage set value with stepwise change to cause the inverter output voltage to fall within an allowable range in accordance with detection of a load current, and a second determination circuit configured to determinate the output voltage set value with stepwise change to cause the inverter output voltage to fall within the allowable range in accordance with detection of a load voltage. Upon grid interconnection, the voltage control circuit updates the output voltage set value with detection of a load current by the first determination circuit, brings a standby state of keeping the output voltage set value from occurrence of power system outage to cutoff of an interconnection switch, brings a standby state of updating the output voltage set value with detection of a load voltage by the second determination circuit since cutoff of the interconnection switch, switches from a current control mode to a voltage control mode, and then switches from the first determination circuit configured to transmit an output voltage command based on the output voltage set value with detection of the load current to the second determination circuit configured to transmit an output voltage command based on the output voltage set value with detection of the load voltage.
Patent Document 2 discloses a power conditioner including a current suppression circuit configured to limit an inrush current due to a load connected to a stand-alone power system so as to be less than a predetermined current value for securement of stable grid independent operation with suppression of the inrush current generated at the load connected to the stand-alone power system.
The inverter device for the distributed power supply disclosed in Patent Document 1 is required to include a dedicated load current sensor configured to measure a load current, to problematically lead to increase in cost for components including a peripheral circuit of the current sensor as well as for installation.
The power conditioner disclosed in Patent Document 2 includes the dedicated current suppression circuit for suppression of an inrush current and is required to include complicated control algorithm applicable to the current suppression circuit, also problematically leading to increase in component cost.
In view of the above, as disclosed in Patent Document 3, the inventors of the present application propose a grid independent operation control unit for a distributed power supply, configured to control to a stable output voltage without provision of any sensor for detection of a load current or any specific component even under a condition with an inrush current.
The grid independent operation control unit is for the distributed power supply including an inverter configured to convert generated DC power to AC power and an LC filter configured to remove a harmonic component from output of the inverter and configured to supply power to a stand-alone power system if a commercial system power supply has a voltage not more than a predetermined voltage. The grid independent operation control unit includes a load current estimator configured to estimate a load current iload supplied to the stand-alone power system in accordance with an output current iinv of the inverter and an output voltage esd, and feedback controls to cause the inverter to output an output voltage command value e*sd in accordance with the output voltage esd and the load current iload.
The load current estimator is configured to estimate a branch current ic flowing to a capacitor of the LC filter in accordance with the output current iinv and the output voltage esd and calculate the load current iload by subtracting the branch current ic from the output current iinv.
The grid independent operation control unit for the distributed power supply feedback calculates to cause the inverter to output the output voltage command value e*sd and calculates a duty ratio u*d for PWM control of the inverter by dividing a result of the calculation by a DC bus voltage Vdc in this case. The duty ratio u*d occasionally fails to fall within the range of −1<u*d<1 due to decrease in DC bus voltage Vdc. There has thus been provided a limiter configured to limit the duty ratio u*d into the range of −1<u*d<1.